The present invention is directed to bicycle brake devices and, more particularly, to a bicycle brake disk that accommodates heat build up during braking.
Disk brake systems are becoming more popular on mountain bicycles because of their superior braking characteristics under severe operating conditions. A typical disk brake system comprises a brake disk that rotates with the bicycle wheel and a brake caliper that sandwiches the brake disk and presses brake pads against opposite sides of the brake disk. The friction between the brake pads and the brake disk causes the braking function. Not surprisingly, substantial heat is generated where the brake pads contact the brake disk during the braking operation. Since the brake pads ordinarily press against the outer radial surfaces of the brake disk, the generated heat is localized at the radially outer portions of the brake disk. As a result of such uneven heating, the radially outer portions of the brake disk expand while the radially inner portions of the brake disk expand very little, if at all. If very high temperatures are generated during the braking operation, this uneven expansion can cause the brake disk to warp. If the brake disk warps to one side, this can cause one of the brake pads to press against one side of the brake disk more than the other brake pad, thus resulting in further uneven heating and a loss of braking efficiency. If the brake disk warps in a wave shape, severe vibration occurs during braking.
One brake disk that addresses the problem of uneven expansion is disclosed in U.S. Pat. No. 5,848,674. That patent discloses a brake disk having an inner ring and an outer ring interconnected by a plurality of spaced arms. Each arm has a curved L-shape so that the stress in the disk caused by the heat generated in the outer ring can be relieved by radial expansion and contraction of the arms. However, the disadvantage of such a design is that the curved arms are less able to resist the rotational forces of the brake disk during braking because of a cantilevering effect at the curves. Also, the curved arms have lower resistance to side impact forces because the curves act as a pivot point for the outer ring relative to the inner ring during the application of such forces.
The present invention is directed to a brake disk of the type having an inner annular portion and an outer annular portion interconnected by a plurality of spaced arms, wherein stress in the disk caused by expansion of the outer annular portion is relieved without compromising rotational and lateral rigidity. In one embodiment of the present invention, a one-piece brake for a vehicle includes a radially inner annular portion having a plurality of circumferentially located mounting holes defining a mounting circle; a radially outer annular portion concentric with the radially inner annular portion and having opposite braking surfaces; and a plurality of connecting arms extending from an inner peripheral surface of the radially outer annular portion to an outer peripheral surface of the radially inner annular portion. Each of the plurality of connecting arms is tangent to an effective circle concentric with the mounting circle, wherein the effective circle has a radius greater than a radius of the mounting circle.
In a more specific embodiment, an inner peripheral surface of the radially outer annular portion defines an outer arm connecting circle, and an outer peripheral surface of the radially inner annular portion defines an inner arm connecting circle. For each of the plurality of connecting arms, a straight phantom line connecting an outer midpoint of the connecting arm on the outer arm connecting circle and an inner midpoint of the connecting arm on the inner arm connecting circle intersects the connecting arm along an entire length of the connecting arm. To improve ventilation of the disk, the radially inner annular portion includes a plurality of openings disposed radially outwardly of the plurality of mounting holes, wherein the effective circle intersects the plurality of openings. Also, the radially outer annular portion includes a plurality of circumferentially disposed vent holes, wherein the plurality of vent holes are arranged to form a plurality of L-shapes.